Microwave integrated circuit (MIC) packages are generally well-known and typically consist of a printed transmission media, e.g., microstrip, on a dielectric substrate within a shielding enclosure. Such devices can be highly useful, but it is obviously necessary that the circuit performance be controlled by the design of the microstrip. However, a problem which is characteristic of MIC packages is that the signals on the microstrip circuitry will radiate to a certain degree within the shielding enclosure. The enclosure will function as a waveguide and undesirable and unpredictable waveguide modes will propagate within the enclosure and interfere with the MIC operation.
The most common method of dealing with this problem is to dimension the cavity such that it functions as a waveguide below cutoff. This can be more clearly understood by referring to FIG. 1 in which an ideal cavity is shown having length L, width a and height b. The cavity 10 contains on its lower surface a dielectric substrate 12. The microstrip circuitry (not shown) will be printed on the surface of the dielectric. It should, of course, be realized that the MIC enclosure will have end surfaces, but these are not shown in FIG. 1. Further, the cavity in FIG. 1 is an ideal cavity, and deviations from the cavity such as reliefs for milling tools will represent minor perturbations.
The problem of undesirable modes resonating within the waveguide enclosure is typically solved by selecting the dimensions of the enclosure such that at least two dimensions are small enough so that the MIC package will operate as a waveguide below cutoff. If only one of the dimensions is below cutoff, it is possible for waveguide modes to propagate within the cavity and interfere with the MIC performance. As shown in FIG. 1, the easiest dimension to minimize is the height b, since the thickness of the MIC board can be extremely small. However, the length L and width a of the package determine the available surface area of the substrate upon which the microstrip circuitry can be formed. In conventional MIC packages the width a is chosen to be below cutoff, but this imposes a substantial restriction on the size of the MIC.
The problem of undesirable modes within the MIC package has been recognized, and several solutions proposed. For example, U.S. Pat. No. 3,863,181 to Glance et al. proposes to provide a groove in the side wall of the enclosure or channel and running around the entire length of the channel. However, such a groove may be difficult to machine. Further, the provision of such a groove would be impractical in existing MIC packages and, therefore, the implementation of the Glance et al. technique would, from a practical viewpoint, necessitate the replacement of all existing MIC packages.
A further solution has been proposed in U.S. Pat. No. 3,936,778 to DeRonde. DeRonde teaches the provision of a conductive member between the substrate and the wall of the enclosure in the vicinity of the launch areas, i.e., in the vicinity of the coax-to-microstrip transitions at either end of the MIC package. However, the theory of operation of the DeRonde improvement is somewhat uncertain. As the length of the cavity increases substantially, it is doubtful that one mode suppressing pin, as shown for example in FIG. 2 of that reference at either end of the enclosure will provide sufficient suppression of the waveguide modes within the cavity.